An amperometric glucose biosensor typically uses a sensor commonly referred to as a “test strip” having at least a pair of electrodes including a working electrode and a counter electrode. The test strip also includes a dry reagent in contact with the working electrode and counter electrode, and a capillary flow channel extending from an inlet opening to the working and counter electrodes. The reagents typically include an enzyme that is capable of oxidizing the glucose in the sample, such as glucose oxidase and one or more mediators adapted to reoxidize the reduced enzyme resulting from oxidation of the glucose, thereby forming a reduced mediator. The test strip is inserted into a meter so that the working and counter electrodes are electrically connected to the components within the meter. After the test strip is inserted in the meter, a sample of a bodily fluid such as blood is introduced into the capillary flow channel and contacts the working electrode, counter electrode and reagent, whereupon the components within the meter apply one or more electrical voltages between the working and counter electrodes, and measure the electrical current passing between the electrodes. The reduced mediator is oxidized at the working electrode, thereby producing a measurable current which is related to the amount of reduced mediator present at the working electrode, and therefore related to the concentration of glucose in the fluid. The measured current typically begins at a high value and then declines and approaches a constant value. For example, the current measured at a predetermined time during application of a voltage may be used to determine the glucose content of the sample.
Users will occasionally attempt to conduct a blood glucose test with a previously used test strip. Such re-use will yield erroneous readings. To prevent re-use, the meter can be arranged to measure conductivity between the electrodes of the strip prior to introduction of a fluid sample. When the strip is inserted into the meter, the electrical components within the meter apply a voltage between the electrodes and measure current flow. A new, unused test strip has only the dry reagent between the electrodes, and therefore has a very high electrical resistance between the electrodes prior to application of a fluid sample. However, a previously-used test strip which is still wet with the previous sample will show a very low electrical resistance between the electrodes and a high current flow. The meter can easily recognize this and emit a warning, terminate the test, or both. However, if the original use of the strip occurred many hours or days previously, the prior fluid sample will have dried. In this case, the conductivity test by the meter will not reveal the problem.
Reuse of previously used, dried test strips can lead to erroneous readings. For example, readings from such used test strip would likely have a very large negative bias due to the loss of chemistry from the working and/or the counter electrode due to the prior use. Thus, further improvement would be desirable.